The role of metal electronic configuration in determining the second-order nonlinear optical response of the homologous series of planar, thermally robust M(salophen) (M = Co, Ni, Cu) transition metal complexes is investigated by electric field induced second harmonic generation experiments and ZINDO quantum chemical calculations. Both the experimental data and those derived from the theoretical calculations (which are in good agreement) indicate that, on passing from closed-shell d(8) Ni(II) to the open-shell d(9) Cu(II) and d(7) Co(II) analogues, hyperpolarizability values increase by a factor of similar to 3 and similar to 8, respectively. These indicate a major role of metal electronic configuration in determining the second-order nonlinear optical response. Partially resonant solution-phase hyperpolarizability values as high as (-170 +/- 40) x 10(-30) cm(5) esu(-1) (Aw = 0.92 eV; mu .beta = 1340 x 10(-48) esu approximate to 2x that for 4-(N,N-dimethylamino)-4’-nitrostilbene) are observed for the Co(salophen) complex. The greater second-order responses of the Cu(II) and Co(II) complexes can be understood in terms of the different natures of the contributing electronic excited states. In particular, the large nonlinearities of Cu(salophen) and Co(salophen) are due to more intense low-energy charge-transfer transitions and the existence of either higher (M = Cu) or lower (M Co) lying metal-to-ligand charge-transfer states. While, for the closed-shell Ni(salophen) complex, the two-state model represents a suitable approximation for describing the nonlinearity, it breaks down in the case of Cu(salophen) and Co(salophen), since other states contribute to the response. Experimental linear and nonlinear optical features are fully consistent with the theoretical calculations.